1. Technical Field
This application relates to storage devices, and more particularly to the field of protecting data stored in storage devices.
2. Description of Related Art
Host processor systems may store and retrieve data using storage devices containing a plurality of host interface units (host adapters), disk drives, and disk interface units (disk adapters). Such storage devices are provided, for example, by EMC Corporation of Hopkinton, Mass. and disclosed in U.S. Pat. No. 5,206,939 to Yanai et al., U.S. Pat. No. 5,778,394 to Galtzur et al., U.S. Pat. No. 5,845,147 to Vishlitzky et al., and U.S. Pat. No. 5,857,208 to Ofek. The host systems access the storage device through a plurality of channels provided therewith. Host systems provide data and access control information through the channels of the storage device and the storage device provides data to the host systems also through the channels. The host systems do not address the disk drives of the storage device directly, but rather, access what appears to the host systems as a plurality of logical volumes. The logical volumes may or may not correspond to the actual disk drives.
It is possible for a logical volume containing application data to become corrupted, either through faulty operation of the application itself or from another source (e.g., improper operation of an unrelated application running on the same host that accesses the same volume). When the data is corrupted, it may be desirable to restore the data to a prior, uncorrupted, state. In systems where periodic backups are performed, the logical volume may be restored from the most recent backup. However, if the time between backups is relatively long, then the prospect of losing a significant amount of data may be unacceptable. For example, if backups are performed daily for a system that handles financial transactions, then restoring a logical volume using backup data means that up to a day's worth of financial transactions may be lost.
One solution to this problem could be to perform backups more regularly. However, many types of backups require copying all the data for a logical volume. Thus, there is a tradeoff between the amount of storage space needed for backup data and the frequency of backups. Moreover, restoring a logical volume from backup data may require taking an application that uses the logical volume off-line while the restoration is occurring, which may not be practical.
Accordingly, it is desirable to provide a system in which the granularity (frequency) of backups may be increased without significantly increasing the need for backup storage while also allowing for applications to operate and modify data during the restoration process.